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The little flat speaker is an electrostatic tweeter, it runs right off the high voltage at the plate of the output tube.
I think at the time this radio was made, seleniums were probably cheaper than the equivalent-rated silicon rectifier, so that's why they were used. Probably the filament voltages add up to enough in this set without another tube, so a tube rectifier could not be used.
Later Zeniths of this type (mid-later 60's) do use a silicon diode.
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